A conventional optical head device is provided with an optical system in which an emitted light beam emitted by a light source is guided to an objective lens to converge at a target position on the recording surface of an optical recording medium such as a CD or a DVD and a return light beam reflected by the optical recording medium is guided to a light receiving element. In the optical head device, adjustment of an optical axis is performed such that various optical elements constructing the optical system are adjustably positioned on a device frame and then fixed on the device frame with an adhesive.
For example, in order to mount a prism 6 for synthesizing and separating optical paths on a device frame 3A as shown in FIG. 5(a) and 5(b), a step portion is formed on the device frame 3A. The step portion is provided with a first positioning plane 31A and a second positioning plane 32A orthogonal to the first positioning plane 31A. The first element face 61 and the second element face 62 of the prism 6 which are orthogonal to each other are respectively positioned by abutting them with the first positioning plane 31A and the second positioning plane 32A. Further, at least the first element face 61 and the first positioning plane 31A are fixed with an adhesive. Consequently, the prism 6 is fixed under the state where the optical axis is adjusted. The device frame 3A is commonly made of aluminum alloy (ADC12).
An optical element such as the prism 6 is normally made of glass but the device frame 3A is made of metal. Therefore, when a variation of environmental temperature occurs, a shearing force acts on the prism 6 from the second positioning plane 32A by the difference of the coefficient of thermal expansion between the prism 6 and the device frame 3A. Especially, when the first positioning plane 31A and the second positioning plane 32A are formed by the step portion of the device frame 3A as the conventional structure where there is no absorbing stress structure provided. As a result, the shearing force is large which acts on the prism 6 from the second positioning plane 32A. Accordingly, when the environmental temperature varies, the position or orientation of the prism 6 may shift and furthermore, may crack the prism 6 or separation of the prism 6 at the adhered face may occur.
Moreover, when the cost of the device frame 3A is reduced by changing the metal material constructing the device frame 3A from aluminum alloy to zinc alloy, such as ZDC2. The coefficients of thermal expansion of the respective materials are represented as follows:
The coefficient of thermal expansion of prism: 7×10−6/° C.
The coefficient of thermal expansion of aluminum alloy: 21×10−6/° C.
The coefficient of thermal expansion of zinc alloy: 27×10−6/° C.
Therefore, the difference of the coefficient of thermal expansion between the prism 6 and the device frame 3A increases. As a result, the above-mentioned problems are even more likely to occur. For example, when the heat cycle tests in the range between −20° C. and +60° C. are performed, the results are shown in FIG. 3(b) which is a graph showing a deviation quantity ΔPX of the optical axis in the X-direction and a deviation quantity ΔPY of the optical axis in the Y-direction. As shown in FIG. 3(b), in the conventional optical head device in which the device frame 3A made of zinc alloy is used, the optical axis of the prism 6 widely shifts when the environmental temperature varies.